1. Field of the Invention
The present invention relates to wavelength division multiplexing (WDM) systems, and more particularly, it relates to WDM systems having a function that a signal light is dropped or added at each repeater or the like in a WDM system.
2. Description of the Related Art
In optical transmission systems for the communication from one end office to another end office and optical transmission systems having a ring path for the optical transmission, there are systems including optical repeaters or nodes. In those optical repeaters, there is an optical repeater having functions of not only amplifying signal lights but also dropping a specific signal light from signal lights transmitted through that node and adding a new signal light to transmitted signal lights.
FIG. 1 shows a prior art ADD multiplexer for adding a signal light to transmitted signal lights. A signal light output at an optical transmitter 5 is sent through an optical divider 3 to an optical coupler 1, in which the signal light is multiplexed and added to WDM signal lights on a transmission line. The other output of the optical divider 3 is input to an output controller 6 after being converted into an electric signal in a photoelectric converter 4. The output controller 6 controls the output of the optical transmitter 5 to keep the input from the photoelectric converter 4 constant. By the above operation, the output of the signal light to be added is kept constant.
In the WDM system in which signal lights having a plurality of wavelengths different from one another are wavelength-division-multiplexed, it is required to adjust the output of a newly added signal light in accordance with transmitted wavelength-division-multiplexed (WDM) signal lights. In the case that the new signal light is added to the WDM signals on the transmission line with an ADD multiplexer, if the peak output of the added signal light is not equal to those of the other channels, the S/N ratio is deteriorated when multi-stage repeats are performed with optical amplifiers. Accordingly, the peak output of the newly added signal light must be equalized to those of the other channels. However, in the system that the transmission output of the optical transmitter is kept constant as the above prior art, a level difference is generated between the newly added signal light and the transmitted WDM signal lights when the peak output of the WDM signal lights on transmission line varies due to a variation of the transmission line loss or the like.